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Understanding U-Values in Sealed Units
Plain-English guide to U-values, what they mean, how they're measured, and how to choose the right sealed unit for Building Regs Part L compliance.
Guide
U-Values Explained
The U-value is the single most important spec for any sealed unit — it measures how much heat the unit loses. Lower is better.
What does a U-value mean?
U-value is measured in W/m²K — watts of heat lost per square metre per degree of temperature difference. A 1.0 W/m²K unit loses half the heat of a 2.0 unit.
Typical U-values
Single glazing
5.8 W/m²K
Standard DGU (no Low-E, air-filled)
2.7 W/m²K
DGU with Low-E + argon
1.1 W/m²K
TGU with dual Low-E + argon
0.6 W/m²K
TGU with krypton fill
0.5 W/m²K
Building Regs Part L (2025)
For replacement windows: max 1.4 W/m²K. For new builds in dwellings: 1.2 W/m²K. Passivhaus: 0.8 W/m²K or lower.